1. Field of the Invention
The present invention relates to an apparatus for rotatably mounting a crank arm in a bottom bracket of a bicycle.
2. Description of the Related Art
A conventional apparatus of this type is known from Japanese Utility Model Publication Kokai No. 1949-102848. This apparatus employs a cylindrical bracket acting as outer races for ball bearings for mounting in a bottom bracket a crank shaft connected to crank arms. According to this structure, spaces in the form of flat rings are defined between end faces of the cylindrical bracket and end faces of bosses of the crank arms. Mud and the like entering through these spaces tend to reach the crank shaft. The crank shaft is a rotary element, and may be worn or deprived of smooth rotation by the intruding mud and the like. Further, wear resulting from the contact between the cylindrical bracket and bearing balls becomes a direct cause of a displacement between the bottom bracket and crank shaft.
Another such apparatus is known from Japanese Utility Model Publication Kokai No. 1989-180393. This apparatus includes a crank shaft unit having a crank shaft connected to bosses of crank arms, a cylindrical element surrounding the crank shaft, and ball bearings provided on inside walls of the cylindrical element for rotatably supporting the crank shaft, the crank shaft unit being mounted in place through adapters. In this structure also, spaces in the form of flat rings are defined between end faces of the crank shaft unit and end faces of the bosses of the crank arms, and intruding dust and mud tend to reach surfaces of the crank shaft to cause wear or impair smooth rotation.